Heretofore, porcelain type dental prosthetic restorations, such as used for crowns, bridges and the like, comprised a restoration consisting of a metallic form or cast which was veneered with a porcelain coating or layer. The metal cast was generally formed of metal such as gold, silver, platinum and/or alloys thereof. Subsequently, the metallic cast portion of a porcelain restoration was made from less expensive base metal alloys of chrome, cobalt, nickel or the like, to which the porcelain was applied and thereafter fired to vitrification under vacuum at temperatures ranging between 1600.degree.-2200.degree. F. While the metal based porcelain restorations were effective, all porcelain restorations are esthetically preferable. However, the forming of all porcelain restorations presented a number of problems distinct from those encountered in making the metal-porcelain restorations.
The making of all porcelain restorations required the making of a die or mold of a suitable refractory material that could withstand the firing temperatures at which porcelain is required to be vitrified. A serious problem heretofore encountered with making all porcelain restorations was the difference in the thermal coefficient of expansion of the refractory material of the die and the porcelain material. This difference in the relative expansion and contraction rate upon heating and cooling of the die or mold and the porcelain cause the porcelain to either separate from the die and/or crack, thereby rendering the predictability of attaining a properly fitting porcelain restoration very uncertain. The problem in achieving a properly fitting all porcelain restoration was further aggrevated when a multiple tooth porcelain restoration was required, e.g., a bridge.
Because of the cracking and/or separation of the porcelain from the die or mold, it was heretofore necessary to effect the necessary repairs after firing and then refire the porcelain. Consequently, with the prior known techniques, repeated repairs and refirings were required. Oftentimes, as many as four, five or more firings were required before a satisfactory all porcelain restoration was achieved. Such numerous firings necessarily increased the overall cost, time and labor involved to fabricate a satisfactory all porcelain restoration.
To obviate some of the problems encountered in making all porcelain type restorations, there is disclosed in U.S. Pat. No. 3,453,756 the concept of utilizing hollow dowels or pins to compensate for the difference of thermal expansion encountered in making all porcelain restorations. It was also noted in this patent that the use of solid pins or dowels was not desirable, as such pins would stress and fracture the refractory die or mold during firing. However, the concept described therein has not, to my knowledge, achieved commercial acceptance and it is not known to be widely used or in use.
It is also known that dowel pins have been widely used in making stone does or molds, which are not subject to firing or used in making porcelain restorations. Dowels used for making stone dies or molds, which are not subjected to high temperatures, are generally formed of plastic or low temperature materials. An example of such dowel as used in a non-firing die or mold is evidenced in U.S. Pat. No. 4,139,943 granted Feb. 20, 1979.
Other examples of pins, posts or dowels as used in various dental procedures, which are not considered to be related to the making of porcelain restorations, are evidenced by U.S. Pat. Nos. 1,639,782; 1,867,300; 3,153,283; 3,541,688; 4,174,570; 4,398,884; 4,443,192; and 4,449,931.